Devices using various types of media such as optical disks, magnetic optical disks, and a flexible magnetic disk are known in the art as disk drives. Among them, hard disk drives (hereinafter referred to as HDDs) have become popular as storage devices for computers to such an extent that they are one of the storage devices indispensable for today's computer systems. Further, not limited to the computers as described above, HDDs are expanding more and more in application due to their excellent properties. For example, HDDs are used for moving picture recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
A magnetic disk used in the HDD has a plurality of data tracks that are concentrically formed. A plurality of pieces of servo data each having address information, and a plurality of data sectors each including user data, are written to each data track. A plurality of data sectors are written between servo data areas. By accessing, according to the address information of the servo data, a desired data sector by use of a head element of a head slider supported by an actuator that pivotally moves, it is possible to write data to the data sector or to read data from the data sector.
In order to improve the recording density of a magnetic disk, it is important to reduce the clearance between the magnetic disk and a head element flying above the magnetic disk. For this reason, some mechanisms for adjusting the clearance have been proposed. In one example of the proposed mechanisms, a head slider includes a heater, and the clearance is adjusted by heating a head element by use of the heater. In this specification, this is called TFC (Thermal Fly height Control). According to the TFC, an electric current is supplied to a heater to generate heat, which causes a head element to protrude. This makes it possible to reduce the clearance between a magnetic disk and the head element. The TFC is disclosed in, for example, United States Patent Publication No. 20050213243 (“patent document 1”). Patent document 1 discloses that a resistance value of a heater element, which changes in response to the temperature, is measured to adjust the output to the heater element so as to compensate for the heater power in response to the value.
If the TFC is adopted, a heater is repeatedly turned ON/OFF several trillions of times during the use of the HDD. Because of this, the long-term reliability of the TFC becomes a major concern. As a result of the study of the inventors, it has been found that if a heater is repeatedly turned ON/OFF by TFC, the thermal expansion and thermal shrinkage cause metal fatigue of materials of the heater, and consequently the breakage of the heater, and a short circuit by electromigration, may occur. In addition, in the boundary between the heater and a surrounding member thereof, a crack may occur due to the difference in material between them. Therefore, it is required to correctly detect or estimate the aging of the heater element, and to perform processing in response to the detection or estimation.